1. Field of the Invention
The present invention relates to a plate-like objects detector for detecting the state of each of plate-like objects (or detecting their presence, number, positional shift from their positions and the like) in the case where the plate-like objects such as semiconductor wafers, LCD substrates and printed boards are aligned in a column and processed as a batch.
2. Description of the Related Art
A plurality of semiconductor wafers are housed in a cassette and the wafers are carried as a lot from a place where a process is conducted to another place where a next process is to be conducted in the course of manufacturing semiconductor devices. 50 sheets of the wafers are batch-processed while immersing them in liquid as a lot in the washing and cleaning system, for example. In order to normally conduct the process, it is needed that the number of the wafers can be kept as previously determined and that they can be aligned as previously expected.
The detecting sensor of the light beam transmission type comprising light emitting and receiving sections was conventionally used as means for detecting the number of wafers and whether or not they are correctly aligned in a column. This detector sensor of the light beam transmission type is disclosed in Japanese Patent Disclosures Sho 61-71383, 61-99344, 61-99345, Japanese Utility Model Disclosures Sho 61-127640, 61-129340 an Japanese Utility Model Publication Hei 1-28683. The sensor disclosed in Japanese Utility Model Disclosure Sho 61-127640 is opposed to each of the wafers arranged in a column and it contains a set of plural light emitting and receiving sections. In the case of this type of sensor, however, the light emitting and receiving sections must be provided same the in number as the wafers to thereby make the sensor device large in size.
In the case of the other conventional sensors, a set of light emitting and receiving sections is operated to scan the wafers in a direction in which the wafers are aligned in a column. However, they must have a mechanism for moving their light emitting and receiving sections to scan the wafers. This makes the sensor device complicated and large-sized. In addition, this mechanism is liable to be corroded in an atmosphere of the liquids which are used in the washing and cleaning system, and to cause particles in clean atmosphere.